1. Field of the Invention
The present invention relates to an electrophotographic toner used for electrophotography, more specifically to an electrophotographic toner that is produced by spray processes, has relatively small particle diameters, and exhibits stable productivity and favorable cleaning ability, and also a method for producing the toner.
2. Description of the Related Art
Higher image quality has been demanded in the fields of copiers, printers, etc. of electrophotographic systems in recent years, and downsizing of toner particles has been vigorously investigated in order to satisfy the demand.
Conventionally, toners have been produced by milling processes in a way that a binder resin, a colorant, etc. are melted and kneaded and the kneaded product is milled and classified. However, the toners produced by the milling processes typically exhibit a broad particle diameter distribution and have a technical limit with respect to the downsizing of toner particles as well as a productive limit with respect to their yields.
Furthermore, so-called polymerization type toners have been investigated recently on the basis of toner production processes such as suspension polymerization processes and emulsion polymerization agglomeration processes. In addition, such processes referred to as polymer dissolving-suspending processes that accompany a volume shrinkage have also been investigated (Japanese Patent Application Laid-Open (JP-A) No. 07-152202). In the processes, toner materials are dispersed and dissolved into a volatile solvent such as organic solvents having lower boiling points, the solution is emulsified to produce droplets in an aqueous medium with a dispersant, then the volatile solvent is removed. The processes are excellent in that resins are widely available and polyester resins in particular, which being useful in full color processes where transparency and smoothness of fixed images are demanded, can be used in contrast to the suspension polymerization processes or emulsion polymerization agglomeration processes.
However, the polymer dissolving-suspending processes are based on the premise that a dispersant is used in an aqueous medium, therefore, there arises such a problem that the dispersant, which typically impairs charging property of toner, remains on toner surface and deteriorate environmental stability and it has also be experienced that a considerably large amount of rinsing water is necessary to remove the remaining dispersant; as such, the processes are not necessarily satisfactory.
A method to produce a toner is proposed as an alternative method for the processes described above, in which fine droplets are produced by use of piezoelectric pulse and the fine droplets are dried and solidified to produce a toner (JP-A No. 2003-262976). Furthermore, a method to produce a toner is proposed, in which fine droplets are also produced by use of thermal expansion within a nozzle and the fine droplets are dried and solidified (JP-A No. 2003-280236); furthermore, a method is proposed, in which similar procedures are carried out by use of an acoustic lens (JP-A No. 2003-262977). However, the toners resulting from these spray processes for producing particles have a truly spherical shape without irregularities at their surface.
When using toners having a truly spherical shape and smaller particle diameters, there arises a problem in cleaning ability of the toners. Blade cleaning is mainly employed in cleaning processes in current electrophotographic systems, and toners having smaller particle diameters and a spherical shape with smooth surface tend to remain on photoconductors without being scraped off by the blades to cause filming. The poor cleaning ability is one of the serious problems in the toners having smaller particle diameters produced by wet processes. It has been found that the condition to have a circularity of no more than 0.98 is necessary to assure the cleaning ability.
Japanese Patent (JP-B) No. 3166369 proposes a method in order to obtain an electrophotographic toner with irregular surface, in which spherical resin particles and inorganic fine particles are dispersed in an organic solvent, capable of swelling and non-dissolving resins, thereby to swell the resin particles and to deposit the inorganic fine particles on the surface of the resin particles, which are then spray-dried to form concave portions on their surface.
In cases of spray processes for producing particles, however, as insoluble dispersing components increase the amounts in ejecting liquids, head clogging tends to occur and also production fluctuation is likely to generate due to selectivity of ejecting components, which making difficult to assure production stability and quality stability.